We already customize everything from cars to computers. This makes them look and behave how we want them to. Scientists now are finding ways to custom-build microbes, too. These miniature life forms might one day be tweaked and tailored to produce food, fuel or life-saving drugs.

That effort just took a big step forward. Researchers report they have just built a synthetic yeast chromosome.

Chromosomes are the threadlike structures in the nucleus of a cell that carry genes. Those genes determine...